The present invention relates to a method for manufacturing a blade made of composite material for a helicopter rotor. The invention applies especially, although not exclusively, to variable-pitch blades intended to equip helicopter tail rotors.
Composite materials offer an attractive solution to the problem of producing helicopter blades with low mass. The technique most commonly employed relies on the use of fabrics which are preimpregnated with a thermosetting resin. This allows various constituent elements of the blade to be assembled and the assembly to be secured together by heating in a mould under pressure (see FR-A-2,616,409 for example or the corresponding U.S. Pat. Nos. 4,892,462 and 4,990,205). This technique provides satisfactory blades, but with a relatively high manufacturing cost, due mainly to the difficulty in mass-producing reliable quality blades, and to the fact that various elements still have to be added to the blade once it is removed from the mould.
An object of the present invention is to propose a method for manufacturing a blade made of composite material making it possible to obtain a quality blade in a reliable and economical manner.
The invention thus proposes a method for manufacturing a variable-pitch composite blade for a helicopter rotor, comprising the following steps:
producing a preform of the blade comprising a blade root section for connection to a hub and an aerodynamically profiled main blade section, the blade root section of the preform including at least one cylindrical portion for guidance in terms of incidence; PA1 arranging a respective peripheral metal ring around each cylindrical portion for guidance in terms of incidence of the preform; PA1 arranging the preform provided with each peripheral metal ring in an injection mould; PA1 injecting a thermosetting liquid resin into the mould; and PA1 heating the mould to make the thermosetting resin set.
This resin transfer moulding technique (RTM) makes it possible to produce a quality blade for a modest cost. In particular, the blade has an excellent surface finish when it leaves the mould, so that very little finishing work is necessary before painting it.
In addition, the blade may already incorporate a certain number of functions when it leaves the mould, especially the function of guidance in terms of incidence by means of the metal rings.
An electrical-insulation tape made of glass fibres and a film of adhesive may be arranged between the preform and the peripheral ring for guidance in terms of incidence. As the resin is injected under a certain pressure, it provides a pressure for bonding the composite material and the ring together. In addition, upon cooling, the metal of the ring has greater thermal shrinkage than the composite material of the preform, so that the ring can be fastened to the composite very robustly. In conventional pressure-moulding methods, it is not generally possible to bond the incidence-guidance rings in this way at the time of moulding, because either the pressure is not applied to the rings, which poses local problems of the integrity of the composite and/or the good bonding of the rings, or the pressure is applied to the rings and tends to deform them.
In the step of injecting the thermosetting resin, the resin is preferably injected into a lower part of the mould, and a partial vacuum is created in an upper part of the mould in order to cause the resin to migrate in the direction of the span axis of the blade. The mould may especially be arranged vertically. This procedure minimizes the risks of air bubbles being present in the blade. Furthermore, since the injection mould is not subjected to substantial positive pressure forces, the constituent elements of the blade do not tend to move around or to deform during injection.
In a preferred embodiment of the method, the preform is produced from dusted fabrics, that is to say ones in which the fibres are not preimpregnated with resin, even though resin (which may be a different one from the resin injected subsequently) is present in the form of powder in the fabric. Such fabrics can be preassembled by moderate heating (of the order of 100.degree. C.) in order to produce a lower shell and an upper shell which are then put together in a heated conforming mould in order to obtain the preform.
The preform may especially include one or more spars, each consisting of a unidirectional tape of carbon fibres, that is to say one which is composed mainly of longitudinal structural carbon fibres, and of a substantially smaller proportion of glass fibres woven transversely between the carbon fibres. These glass fibres give the spar mechanical strength for handling it for producing the preform, and also serve to form gaps between the longitudinal carbon fibres allowing the resin to flow during the injection phase.
As a preference, the preform includes, in the blade root section, a pile of fabrics transverse to the span axis of the blade and projecting with respect to the blade root section, so as to constitute, after injection, a pitch-control lever. The pitch-control lever may thus be incorporated right from the time of moulding. Its positioning is accurate and reproducible because it is determined by the shape of the injection mould, this eliminating the dimensioning and positioning problems usually encountered when the pitch-control lever is added on after the blade leaves the mould.